1. Holding down Ctrl will make a copy of the clip.
2. Holding down Shift locks either the horizontal or the vertical position depending on which way you move the mouse - i.e. if you move the mouse vertically, then the time position is locked but you can move the clip to other tracks; if you move the mouse horizontally, the clip is locked to that track.
Holding down Ctrl + Shift will preserve the behaviour of (2) above, but will make a copy of the clip rather than moving it.